Often, semiconductor devices include data converters, such as analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). The data converters are used in applications that include digital signal processing of real-world signals, such as applications in audio processing, video processing, instrumentation, and communication systems. Applications include mobile phones, base stations, cordless networks, imaging systems, test equipment, and radio frequency (RF) transceivers. Each of the semiconductor devices can be a single integrated circuit chip or multiple integrated circuit chips.
An ADC converts an input analog signal to an output digital signal, where the value of the output digital signal corresponds to the amplitude of the input analog signal. The output digital signal can be a series of digital code words or a serial bit string.
A DAC converts an input digital signal to a corresponding output analog signal. Usually, digital codes are converted to analog signals by assigning a voltage or current weight to each bit in the digital code and summing the weights of the digital code.
Typically, a DAC includes a decoder, a number of analog output elements, and a summing circuit. The decoder receives an input digital code and provides selection signals to selectively activate the analog output elements. In response to the selection signals, selected analog output elements provide analog signals that are combined by the summing circuit to produce the analog output, which is an analog representation of the input digital code. The analog output elements can be unity coded or weighted. In a DAC including unity coded analog output elements, such as a thermometer-coded DAC, each of the selected analog output elements provides the same amount of voltage or current. In a binary-weighted DAC, the analog output elements provide amounts that are weighted by the power of two. Sometimes a DAC is segmented to include a thermometer-coded portion and a binary-weighted portion.
Often, in a current-steering DAC the analog output elements are current cells, where each current cell includes a differential switch and a current source. The differential switch is controlled to steer current from the current source to the output of the current cell or to another node, such as ground, based on the input digital code. The outputs of the current cells are combined to yield a total current that is proportional to the number of switched-on current cells. The total current can be transformed into a voltage via resistors and amplifiers. In a current-steering DAC, the current sources are never shut off and power is continuously consumed by each of the current cells.
In a DAC including current cells that have a single-ended switch and a current source, the switch is controlled to provide the current to the output of the current cell or to shut off the current based on the input digital code. This reduces power consumption. However, if switches are not turned on and off simultaneously, current sources in different current cells can short together and parasitic capacitances can be discharged. Each of these events leads to glitches in the analog output and poorer performance.
For these and other reasons there is a need for the present invention.